Various types of solar furnace heating systems have been disclosed in the patented prior art, as evidenced by the patents to Thompson U.S. Pat. Nos. 2,931,578, Morgan 3,243,117, Schoenfelder 3,863,621, Keyes et al. 3,894,685 and Parker 3,919,998.
It is conventional in the solar furnace art to provide heat collector elements of the flat plate type including a planar metallic plate which is selectively coated to act as a black body solar energy absorber/heat emitter. In an air transfer medium collector, the air travels through the collector chamber in a path parallel to the planar surface. This flat-plate collector has proven to be inefficient because the black body absorber absorbs only about 90% of the solar energy entering the chamber, thereby reflecting some 10% of the solar energy out of the collector since there are no light entrapment qualities. Another inefficiency of the flat-plate solar collector arises from the fact that the air transfer medium passes over and parallel to the flat collector plate so that little air turbulence is created which reduces the amount of heat energy transferred to the air transfer medium.
An improvement over the flat-plate type solar collector is disclosed in the aforementioned Keyes et al. patent wherein a vertical plate collector is provided with extruded aluminum cups on the surface thereof, the cups having walls perpendicular to the base. The vertical plate collector having a plurality of cups on its surface provides a number of improvements over the flat-plate collector. First, increased energy absorption is provided in that as a ray of solar energy enters a collector cup, it is subject to a number of "bounces" off of the surfaces of the cup. With each bounce, some energy is absorbed by the cup in the form of heat. Second, the effective surface area of the collector surface is approximately four times greater than the surface area of the prior flat-plate collector surface for a given solar window. Third, as forced air is passed over the openings of the cups, a Venturi effect causes a turbulent action to draw heat off of the cups for increased heat exchange to the air transfer medium.
Some inefficiencies, however, do arise form the cupped-collector surface of the apparatus of the Keyes et al. patent. The collector cups do not provide energy entrapment qualities. The bouncing of the solar rays does increase energy absorption, but eventually some solar energy is reflected back out of the collector. Also, the insolation strikes the side walls of the cups at acute angles which provide less energy absorption than insolation which strikes the walls at angles approaching 90.degree.. Finally, while some turbulence causing air impingement is created, it is far from ideal. For maximum heat transfer, the air mass should impinge the collector surface directly at right angles but with enough turbulence so as to prevent a build up of a thin, stagnant, insulating air layer.